ARGUMENTS rfn The function to be called to process
requests that have been placed on the queue.

lock Request queue spin lock

DESCRIPTION If a block device wishes to use the standard
request handling procedures, which sorts requests and
coalesces adjacent requests, then it must call
blk_init_queue. The function rfn will be called when there
are requests on the queue that need to be processed. If the
device supports plugging, then rfn may not be called
immediately when requests are available on the queue, but
may be called at some time later instead. Plugged queues are
generally unplugged when a buffer belonging to one of the
requests on the queue is needed, or due to memory
pressure.

rfn is not required, or even expected, to remove all
requests off the queue, but only as many as it can handle at
a time. If it does leave requests on the queue, it is
responsible for arranging that the requests get dealt with
eventually.

The queue spin lock must be held while manipulating the
requests on the request queue; this lock will be taken also
from interrupt context, so irq disabling is needed for
it.

Function returns a pointer to the initialized request
queue, or NULL if it didn´t succeed.

NOTE blk_init_queue must be paired with a
blk_cleanup_queue call when the block device is deactivated
(such as at module unload).